Setting tools are used in oil and other types of wells to "set" downhole devices such as packers and bridge plugs. Setting involves fixing or securing the device within the hole at a desired depth. To set a packer or bridge plug, it is coupled to a lower end of a setting tool at the surface using a frangible connection and then lowered down the well on a wire or cable. Once lowered to the desired depth, an outer sleeve of the tool is moved relative to an internal rod or mandrel to create a shearing action. This shearing or "setting" action compresses the packer or plug. Compressing the packer or plug, which is primarily composed of rubber or like materials, forces gripping slips on the outside of the packer or plug into the well casing, thereby setting the plug or packer. Once the packer or plug is set, an extra shearing force is applied to break the frangible connection to disconnect the setting tool for retrieval.
The force required to set the packer or plug, termed the setting force, depends on the size of the downhole device and hole. However, no matter the size, the force that is required is quite substantial given that it must be generated within the relatively small confines of a bore hole. In most setting tools, the setting force is generated by detonating an explosive charge or bomb within a firing chamber of the tool. Exploding gases compress hydraulic fluid, which in turn drives a piston that actuates the tool by moving its outer sleeve relative to its inner mandrel.
Using an explosive charge to actuate a setting tool has several problems. These problems are described in U.S. Pat. No. 4,493,374 of Robert L. Magee, Jr., titled "Hydraulic Setting Tool", which issued Jan. 15, 1985. One disadvantage is that explosives are dangerous, and their handling at drill sites slows down, complicates and increases the costs of setting down hole devices. A second disadvantage is that the firing chamber must also be cleaned between uses, thereby adding to the maintenance of the tool. A third disadvantage is that an explosion cannot be well controlled to provide a gradual actuation of the tool in order to allow elastomers and highly elastic materials of the plug or packer to respond and conform better to the contours of the well bore, thereby providing a better seal. Slow burning explosives and orifices through which hydraulic fluid is forced during the explosion have been used to slow down the sudden, initial force of the explosion. However, these methods do not result in a setting force that is constant. The setting force starts at a maximum at the beginning of the setting action or stroke of the tool and substantially drops along the setting stroke of the tool.
To overcome these disadvantages, U.S. Pat. No. 4,493,374 of Magee proposes using a motor to drive a radial piston pump that pressurizes hydraulic fluid. The pressurized fluid acts against and moves a piston portion of the setting tool. Movement of the piston causes the outer sleeve to move downwardly with respect to the inner rod. Thus, the hydraulic pressure thus may be gradually applied. This type of setting tool also has several other advantages. The stroke of the tool is not limited by the shortness of explosive force since the electric motor may continue to function as long as is necessary to supply enough hydraulic fluid to move the piston the desired length. And, because the tool need not be purged, cleaned and re-loaded with an explosive charge, it may make several successive runs very quickly. However, despite having many advantages over setting tools using explosive charges, the setting tool disclosed in Magee '374 has several shortcomings not previously noted which prevent it from operating as well as desired.
A hydraulic pump requires a minimum head or pressure at its intake, sometimes referred to as its low pressure side, in order to properly pressurize the fluid and avoid cavitation, as well as to ensure enough fluid around its intake to avoid entrainment of air. The Magee '374 setting tool uses a reservoir located above the hydraulic pump and below the motor to store fluid that will be pumped into the setting chamber for actuating the tool. The head on the low pressure side of the pump is thus simply the weight of the oil. As the temperature of the hydraulic fluid rises, the weight of the fluid decreases and the fluid becomes more susceptible to entrainment of air. Tilting of the reservoir when the tool is conveyed down a slant-bore exacerbates the threat of pump cavitation.
The failure to maintain viscosity of the hydraulic fluid and the propensity to pump cavitation of the Magee '374 setting tool are very undesirable attributes. These drawbacks not only decrease the life expectancy of motors and pumps, which are expensive due to the very large pressures they are required to develop, but also interrupt application of the setting force and cause premature failures.
The Magee setting tool has a further shortcoming in that the setting force it is capable of generating is not sufficient for some deep well applications. Although larger motors could be used to generate higher pressures on the hydraulic fluid, the bore hole limits the size of the motor.
It is therefore the object of the invention to overcome these problems and the very inhospitable conditions found in deep wells which affect the operation of hydraulically activated setting tools. It is the further objective of the invention to improve this type of setting tool with additional features to render it more useful in the field.